A. Field of the Invention
This invention relates to a method of treating a subterranean formation, containing silica, clay, or mixtures of silica and clay, of nonuniform permeability for the purpose of producing hydrocarbons from the subterranean formation, and more particularly, concerns a method of reducing the permeability to water of a subterranean formation penetrated by a well bore, and containing silica, clay, or mixtures of silica and clay, thereby reducing the quantity of water recovered from the subterranean formation and increasing the relative production rate of hydrocarbons from the formation.
B. Description of the Prior Art
The production of large amounts of water from oil wells and gas wells constitutes one of the major items of expense in the overall recovery of hydrocarbons from a subterranean formation. Many oil wells will produce a gross effluent comprising greater than 80% by volume water. Most of the pumping energy is therefore expended in lifting water from the well, and thereafter the production effluent must be put through expensive separation procedures in order to recover water-free hydrocarbons. The remaining water constitutes a troublesome and expensive disposal problem. It is therefore highly desirable to decrease the volume of water produced from oil and gas wells. By decreasing the flow of water into the well bore, another beneficial effect is obtained in that, at a given pumping rate, there will be a lower liquid level over the pump in the well bore, thus reducing the back pressure in the formation and improving pumping efficiency and net daily oil production.
Undesirable water, including brines, recovered from a well bore penetrating a subterranean formation can result from the infiltration of naturally-occurring subterranean water or, in the case of waterflood projects, from injection of brines or water. Either source of water leads to lowered efficiency in producing the desired hydrocarbons from the formation. In the application of waterflooding to hydrocarbon reservoirs, poor sweep efficiency often leads to disappointing results.
Anionic polymers, such as polyacrylamide, have been used in the past to reduce the amount of water production from oil or gas wells by means of injecting a solution of the polymer into a subterranean formation thereby reducing the effective water permeability of the formation. For example, U.S. Pat. No. 3,308,885 discloses introducing an aqueous solution of a water-soluble polyacrylamide polymer into a subterranean formation around a well bore to reduce the water/oil ratio of the formation effluent. The polymers have also been used to change the injectivity profile of water injection wells in order to make more efficient use of the water and to change the mobility of the injected water.
The polymers alter the permeability to water of the subterranean formation by absorbing into the surface of the rock in the formation. Although the polymers have been effective in reducing the amount of water recovered from a subterranean formation, certain problems remain. For instance, the polymers, while effective, have limited permanence which results in a treatment with a limited lifetime. It has been found that, when a subterranean formation has been treated with the polymers to reduce water production, substantial amounts of the polymers are removed from the formation by fluids that are produced or injected into the formation necessitating retreatment of the formation. Polymers that are not adequately attached to the surface of the rock are particularly susceptible to removal by these fluids. The use of a crosslinking agent with the polymer in such a treatment has not been completely successful in stabilizing the polymer in the formation and preventing its loss after treatment. An indicator of the instability of the polymers has been the results of treatments of producing wells over extended periods of time. In some instances, the benefits of the polymer treatment in reducing water production has been lost within three to six months or even sooner after the initial polymer treatment.
By the present invention, a method of treating a subterranean formation, containing silica, clay, or mixtures of silica and clay, is provided which improves the stability and long term effectiveness of anionic polymers which are utilized to reduce the permeability to water of the subterranean formation.